1. Field of the Invention
The present invention relates to a technique for inspecting a circuit board on which various parts are packaged (hereinafter, referred to as “printed circuit board”) for any defect such as a disconnection and a short-circuiting of a wire pattern, a defective part, a positional deviation, a raised state of a part, insufficient and exceeding soldering processes and a breakdown of a part.
2. Description of the Background Art
With respect to inspections of this type, conventionally, an inspection using an in-circuit tester has been carried out. In this inspection, as shown in FIG. 20, after an electric current has been applied to a printed circuit board 4, two points are selected from a wiring pattern 7 and terminals 6a, 6b of a part 6 to be inspected, and test probes 31, 32 are applied to these points so that factors, such as an electric current, a voltage, an impedance or a frequency, are measured between the two points with which these probes 31, 32 have been made in contact, and when the measured value is an abnormal value, it is determined that there is any defect between the above-mentioned two points.
In addition to this method, an inspection using a visual recognition device is sometimes carried out. In this inspection, while inspection areas are being successively set on a circuit board to be inspected, respective images are picked up so that, based upon the state of the resulting image, a determination is made as to whether or not any defect exists therein; and devices for carrying out inspections of this type are classified into visual inspection type devices in which an inspector visually inspects the images so as to determine the quality of the circuit board, and automatic inspection type devices in which, based upon predetermined reference data, a computer determines whether or not there is any defect.
Along with the recent developments in printed circuit boards with high-density, extremely fine parts and wiring patterns, it has become difficult to correctly put test probes onto portions to be inspected in the case of inspections using an in-circuit tester. Moreover, when test probes are put onto a printed circuit board having a high density, the circuit board might be damaged due to a disconnection in the wiring pattern. In the case of a circuit board used for an apparatus requiring a high degree of safety, a redundant circuit such as a duplexed circuit is formed therein, and with respect to such a redundant circuit, test probes are put at a position before the branch of the respective circuits and at a position after the branch; therefore, even when there is any defect in one portion of the pattern, if the other pattern is normal, the detected electrical potential by the test probes shows the same value as the normal value, thereby failing to properly detect a defect.
Here, in the case of the inspection using a visual recognition device, when a defective portion is extremely small or a degree of a defect is very small, the defect might be ignored. Moreover, with respect to parts that have been packaged by using a face-down packaging method, since connecting portions cannot be recognized from the face side of the circuit board, it is impossible to carry out the detecting process by using a visual recognition device.